The present invention relates to unidirectional streaming services (e.g. voice mail and Internet audio/video distribution) in wireless systems, with particular applications for improving the reception quality of unidirectional streaming services data.
In digital wireless systems there exists a need to support stream based services. Stream based service are services where information is delivered to the user while data is being received, rather than waiting for all of the information to be received before commencing delivery, for example, voice or video clips. In principle, stream based services cannot accept lengthy or variable delays in transmission. For example, for a normal interactive voice call, the speech signals are transmitted and delivered with as little delay as possible, in order to avoid upsetting the natural conversational dynamics. Typically, in order to ensure real-time delivery, stream based services have required dedicated communication channels (e.g., circuit switched calls, which are typically full duplex) in order to avoid lengthy delays in transmission. Recently, streaming services data, for example voice over IP, have been transmitted using packet services which require quality of service (Q of S) controls to avoid lengthy and variable delays.
However, for conventional packet data sent by wireless transmission, receiving uncorrupted data is typically considered sufficiently important to allow for delays in order to retransmit corrupted data. Thus, error detection and Automatic Repeat Request (ARQ) protocols are used to ensure accurate transmission of data. These retransmissions cause delays which are unacceptable for real time delivery of speech and other stream-based services.
The inability to wait for the retransmission of corrupted data is a problem which affects the quality of the received voice/video data in wireless transmission systems, as a result of drop-outs and short interruptions of the radio channel caused by interference, multipath fading and blocking, etc.
Furthermore, conventional digital wireless systems have handled unidirectional streaming services (for example, voice-mail playback and internet audio/video distribution) similar to bi-directional circuit-switched voice services.
FIG. 1 is an example of a wireless system retrieving a voice message (a unidirectional voice streaming service) in a conventional manner. This system treats the voice message in the same manner as real time, interactive speech. First, the incoming voice signal from the voice-mail server is organized into speech frames according to the specific speech encoding technique used in the air interface. These speech frames are then transmitted in a manner which is well known in the art. Following the particular air interface speech coding and processing technology, which is also well known in the art, the received frame containing an error is reconstructed by extrapolating the preceding speech frame(s). In this example, the error detection means determines that there is an error in frame 2. In this example, frame 2 would be reconstructed to the extent possible by extrapolating frame 1.
This conventional method of providing unidirectional streaming services is susceptible to the above mentioned wireless transmission impairments. While extrapolation of corrupted data often improves quality compared to simply converting the corrupted data into speech, the speech quality is still degraded.
There exists, therefore, a need for improving the reception quality of unidirectional streaming services data in wireless systems.
It is an object of the present invention to provide a method for improving the reception quality of unidirectional streaming services (USS). Note that this is expected to be particularly advantageous in wireless systems and this specification will discuss specific embodiments for wireless networks. However, the invention can also be used in other networks which transmit USS data, for example, internet audio/video distribution.
According to one aspect of the present invention this objective is met by storing received USS data frames in a buffer prior to commencing delivery to the user application or recipient. Note that for the purposes of this document, received data refers to the data stream as received over the communication channel. The individual packets comprising this data stream may not be received at regular intervals, or even in the same order as they were sent. Delivered data refers to the data stream finally delivered to the user (often via some intermediate application for example a speech decoder). In this case, the individual packets will have been re-assembled into the correct order, and any irregularities in inter-packet timing corrected.
Advantageously, the inventors have recognized that although frames must be delivered at a constant rate, for example, in order to maintain speech dynamics, an initial delay prior to initiating delivery will typically be acceptable. This fact can be exploited to improve the reception quality by storing a plurality of received frames in a buffer prior to delivery, and using the delay introduced by the buffer to allow for retransmission to replace frames received with errors. Depending on the system and its capacity, this can delay the delivery of the USS data, which until now has been considered unacceptable according to conventional thinking regarding USS data, such as audio or video data. The inventors have determined that the limitations which prevent delays in interactive, real-time, streaming services data, for example a typical live telephone conversation, are not the same for unidirectional streaming services. For an interactive telephone call, no significant delay can be accepted as either party can speak at any moment in time. Therefore, all data must be transmitted in substantially real-time as there is no predictability as to when a user will speak. This has typically required a circuit switched connection, which are typically full duplex.
However, the inventors have recognized this does not hold true for unidirectional streaming services. The data must be delivered at a constant rate (e.g., a normal talking rate for voice data), but as the service is undirectional, its delivery requirements are predictable. The inventors have determined that, by storing received frames in a buffer prior to the delivery of the data, the transmission rate need not be the same as the delivery rate. Thus, the addition of a buffer can be used to improve reception quality, by providing the system with time to replace USS data frames received with errors (or not received at all due to frame loss in the transmission medium). For example, the buffer allows for the retransmission of corrupted data by using an ARQ protocol. Advantageously, such a system will require less voice frame reconstruction than conventional systems.
In operation, let us look at the example of a wireless network transmitting USS data to a wireless terminal. A high layer protocol organizes the USS data into frames and said data is transmitted to the wireless user""s terminal. The USS data is not delivered until a criteria is satisfied, e.g., the receive buffer is filled to an appropriate value. After the USS data is received, the receiver verifies the received frames. If an error is detected a message is sent from the wireless terminal to the server (providing the unidirectional streaming service) requesting retransmission of the corrupted frame. This is a form of ARQ protocol. If the retransmitted frame arrives prior to the time that frame needs to be delivered, the corrupted frame is replaced by the retransmitted frame. Otherwise, if the retransmitted frame is not received prior to the time that frame needs to be delivered, the corrupted frame is reconstructed. Any retransmitted frame which arrives too late is discarded. An additional benefit of the present invention is that since the frames are buffered prior to delivery, interpolation, as opposed to extrapolation, can be used to improve the reconstruction quality of the corrupted frame. This optional feature of the invention uses both the preceding and succeeding frames, which are available in the buffer, to provide a better estimate of the corrupted frame.
Another aspect of the present invention is that, as the USS data will be stored in a buffer of the wireless terminal prior to delivery, the USS data does not have to be transmitted over the wireless interface at its delivery rate (e.g., a normal talking rate). Preferably, the transmission rate is faster than the constant delivery rate. Preferably, for wireless systems which can use variable transmission rates, the USS data can be transmitted as packets with a variable transmission rate, which will typically conserve bandwidth compared to a full-duplex circuit switched connection. Thus, the transmission rate can be varied, sometimes transmitted at a faster rate and other times sent at a slower rate, or with momentary pauses. Preferably, the average of said variable rate exceeds the constant delivery rate. Furthermore, the transmission rate can be varied depending on the short-term traffic on the wireless channel in order to avoid system overload. Preferably, in order to quickly fill the receive buffer with a desired number of frames and make the delay less noticeable to the user, the data is initially transmitted at a faster rate than said constant rate, assuming the data is initially available. Furthermore, and even if the data is only supplied to the transmitter at the constant delivery rate, a variable transmission rate can be used to quickly replenish the receive buffer in a buffer underflow situation, i.e., if the number of frames in the receive buffer drops below a threshold (for example, as a result of multiple transmissions).
Note that this system introduces a buffer delay between receiving a number of frames and delivering said number of frames to a recipient user as the frames are now not directly delivered when received. Note that the term buffer delay does not necessarily imply that a constant delay is applied to each frame. In fact, if a variable transmission rate is used, the time successive frames wait in the buffer prior to delivery will vary.
As an optional feature, outgoing voicemail message are stored in a transmit buffer prior to transmission so that silence deletion can be applied. This is advantageous since it will reduce the amount of speech data sent over the wireless channel.
According to one aspect of the invention, there is provided a method of delivering unidirectional streaming services (USS) data transmitted via a wireless network, said method comprising the steps of:
a) storing received USS frames in a receive buffer;
b) testing received USS frames for errors;
c) replacing, USS frames received with detected errors prior to delivery; and
d) delivering said USS frames from said buffer at a constant rate responsive to a delay criteria being satisfied;
wherein said replacing step comprises:
i) requesting retransmission of said USS frames received with detected errors;
ii) replacing the USS frames received with detected errors with retransmitted frames provided the retransmitted frames are received without errors in time for delivery; and
iii) if said retransmitted frames are not received without errors in time for delivery, reconstructing the USS frames received with detected errors and discarding any subsequently received retransmission of said frames.
According to another aspect of the invention there is provided a method of delivering unidirectional streaming services (USS) data comprising the steps of:
a) determining whether said USS data requires a symmetrical, bidirectional communication link with a recipient user; and
b) responsive to said determining step determining said USS data does not require such symmetrical, bidirectional communication link with a recipient user, introducing a buffer delay between receiving a number of frames and delivering said number of frames by:
i) receiving in a buffer USS data transmitted with a variable transmission rate; and
ii) delivering said USS data with a constant delivery rate.
Other aspects of the invention include associate transmission methods, and transmitters, receivers, wireless terminals and networks for carrying out the new methods, and voice mail systems for using such systems.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.